Under the 3GPP standards, a NodeB (or an eNodeB/eNB in the case of the so called ‘Long Term Evolution’ or ‘LTE’) is a base station via which mobile devices connect to the core network. Historically, base stations comprised ‘macro’ base stations which provide communication cells having a relatively large geographical coverage. More recently, however, base stations having a more limited geographical coverage have also been standardised to provide improved coverage in geographical areas where a macro base station's coverage is known to deteriorate and/or to provide additional services targeted at specific users or groups of users.
Specifically, for example, the 3GPP standards body has adopted a standard for so called ‘home’ or ‘femto’ base stations which provide so called ‘femto’ cells having a relatively limited geographical coverage. Where the home base station is operating in accordance with the LTE standards, it is sometimes referred to as a HeNB. The home base station is also sometimes referred to as a femto cell, especially in the case of the so called WiMAX network. Typically, a home base station provides a radio coverage (for example, 3G/4G/WiMAX) within a limited geographical area, often within a building, for example in a home, a small or medium sized enterprise, a shopping mall etc., and connects to a core network via a suitable public network (for example via an ADSL link to the Internet) or operator network.
Optionally, the radio coverage provided by the home base stations may be supported via a ‘home (e)NodeB gateway’ (HNB-GW) which typically aggregates traffic from several home base stations and handles communication between the home base station(s) and at least one mobility management entity (MME) in the core network. The HNB-GW is, in effect, transparent to the MME and the home base station with the HNB-GW appearing to be a mobility management entity (MME) from the perspective of the home base station, and to be a standard base station from the perspective of the mobility management entity (MME).
A home base station can generally be configured according to any of the following access modes:                a ‘closed’ access mode in which the home base station operates a closed subscriber group (CSG) cell to which only members of the CSG may be provided access;        a ‘hybrid’ access mode in which the home base station operates a CSG cell to which members of the CSG may be provided preferential access whilst non-members are allowed non-preferential (non-CSG) access (for example with a different charging structure, with a different level of service, and/or subject to CSG members being able to successfully access the cell during high load scenarios); and        an ‘open’ access mode in which the home base station operates as a normal (non-CSG) cell to which access is provided openly.        
It has also been decided, as part of the 3GPP standardisation process, that downlink operation for system bandwidths beyond 20 MHz will be based on the aggregation of a plurality of component carriers at different frequencies (this is referred to as carrier aggregation). Such carrier aggregation can be used to support operation in a system both with and without a contiguous spectrum (for example, a non-contiguous system may comprise component carriers at 800 MHz, 2 GHz, and 3.5 GHz).
Carrier aggregation can be particularly beneficial in a heterogeneous network (HetNet), even when the system bandwidth is contiguous, and does not exceed 20 MHz because multiple carriers enable interference management between different classes of cells (e.g. between a macro cell of a macro base station and femto cells operated by a home base station) and also between open access, hybrid and closed subscriber group (CSG) cells operating in the same geographical vicinity.
In accordance with carrier aggregation, therefore, a macro or home base station can operate a first ‘primary’ cell (PCell) using a first component carrier and at least one further ‘secondary’ cell (SCell) on a further component carrier. Further, in accordance with operation of home base stations, a home base station can operate a PCell using a first component carrier and having a first access mode (closed, hybrid or open) and an SCell using a second component carrier and having a second access mode (closed, hybrid or open), which second access mode may be different to the first access mode. A home base station can also operate a PCell having a hybrid or closed access mode with an associated first CSG comprising a first set of members and an SCell having a hybrid or closed access mode an associated second CSG comprising a second set of members, which second set of members may be different to said first set of members.
Thus, the use of open, hybrid, and closed access mode cells, in combination with carrier aggregation, has the potential to provide an operator of a home base station with great flexibility to operate different cells for different groups of subscribers—potentially with different charging policies for each cell. For example, a home base station operator could, theoretically, set up their home base station to provide a PCell to a relatively large CSG having a relatively large number of members, and an SCell to a smaller CSG, comprising a subset of the relatively large CSG (e.g. whose members are willing to pay a premium to have access to the additional resources of the SCell). Several other similar scenarios are, of course, possible with different cells (primary or secondary) serving distinct or overlapping subscriber groups or with one cell serving any user (i.e. open access mode), another serving only members of a particular CSG (i.e. closed access mode), and/or another serving members of the CSG preferentially (i.e. hybrid access mode) but allowing other users in when sufficient resources/quality of service are available.
However, there are still a number of technical issues with the use of open, hybrid, and closed access mode cells, in combination with carrier aggregation, which need to be addressed if the above flexibility is to be fully realised. Firstly, for example, the aggregation of one component carrier with one or more others to provide a PCell and one or more SCells is based on radio conditions. Hence, a home base station that operates a PCell and an SCell may make a decision to initiate the configuration of user equipment that is connected via the PCell for use of the SCell (referred to as SCell addition) based on radio condition alone (e.g. responsive to a measurement report from the user equipment. Thus the user equipment of a subscriber to a CSG of a closed/hybrid access mode PCell may be configured to use a closed access mode SCell having a CSG to which the user is not a subscriber.
Another potential issue arises when the different cells (PCell/SCell(s)) via which the user equipment is connected (or is about to be configured to use) have different respective CSGs for which different respective charging structures apply. In this situation, there is currently no efficient way to determine which charging structure should take precedence over another, and a user is unaware of how conflicts of charging structure will be dealt with (e.g. will they be charged the maximum or minimum charge that applies across all the cells of the aggregated component carriers).
Yet another potential issue may arise, for example, when a home base station operates a closed/hybrid access mode PCell having a first CSG, and a closed/hybrid access mode SCell having a second CSG, and the user equipment of a subscriber to both the first CSG and the second CSG is connected in the PCell and the SCell. In this situation, if the subscriber's subscription to one of the CSGs expires, whilst the other remains valid, the component carrier (cell) configuration in the subscriber's user equipment no longer complies with a valid CSG list and the user equipment may continue to use resources of a CSG of which the subscriber is no longer a member (a similar issue arises when user equipment is connected to a plurality of SCells and a CSG subscription expires in respect of one of those SCells).
Moreover, when a handover is required from a source home base station operating primary and secondary cells having a first set of CSGs, to a target home base station operating primary and secondary cells having a different set of CSGs, there are potential issues in how such a handover can be accomplished efficiently whilst maintaining appropriate access controls and membership checks.